These systems used in air conditioning are operated in accordance with a thermodynamic cycle which is defined in general by an evaporation temperature of between 0.degree. and 10.degree. C. (most often 7.degree. C.), a condensation temperature of between + and 55.degree. C., a liquid subcooling of the order of -5.degree. C. and a vapor superheat of at least 10.degree. C.
The refrigerant fluid currently used in air conditioning is chlorodifluoromethane (called HCFC 22). However, it is now established that, because of their coefficient of reaction with ozone, HCFCs--and in particular HCFC 22--will sooner or later have to be replaced by refrigerant fluids which no longer contain chlorine and which, for this reason, are less aggressive with regard to the environment.
In order to replace HCFC 22 in existing air conditioning installations, the substitute must have thermodynamic properties--in particular a coefficient of performance (COP) and a refrigerating capacity--which are as close as possible to those of HCFC 22.
Moreover, for the good stability of the product and the durability of the material, it is desirable that the discharge temperature does not exceed that of HCFC 22 by more than about 5.degree. C. Finally, the substitute must be nonflammable and remain so should the vapor phase leak.
In comparison with chlorinated compounds, difluoromethane (HFC 32), tetrafluoromethane (FC 14), octafluoropropane (FC 218) and 1,1,1,2-tetrafluoroethane (HFC 134a) show no reaction with ozone and have very little action on the environment.
HFC 32 has the major disadvantage of being flammable. Moreover, the fact that its boiling point is considerably lower than that of HCFC 22 means that it is not suitable for the direct replacement of HCFC 22. Those skilled in the art are aware that a low boiling point involves very high pressures, which render the use of such a fluid impossible in existing systems, for reasons of safety.
HFC 134a, a nonflammable compound, is a good refrigerant fluid in terms of efficiency, but its deficient refrigerating capacity renders its use as a substitute for HCFC 22 impossible.
Mixtures of the two abovementioned HFCs in certain proportions give a good compromise between efficiency and refrigerating capacity but, in these proportions, the mixtures have a vapor phase which is flammable.